It is a requirement in all vehicles to match variable road load demands, such as speed and torque, to the output of the engine. The engine operating profile has areas of greater and lower efficiency and a given power level (which is the product of torque and speed) can be delivered with a number of different combinations of engine torque and speed that can give greater or lower efficiency and hence greater or lower fuel consumption and emissions. This has historically been achieved in two ways: either with a discrete number of gears, engaged manually by driver or automatically by a control mechanism; or with a continuously variable transmission (CVT) system employing mechanical elements such as cone-drive systems. The latter does not have fixed steps in gearing.
One of the ways that electric hybrid vehicles aim to increase fuel efficiency and lower emissions is to use a combination of energy sources (i.e. battery and fuel tank) and a combination of electrical and mechanical drives to ensure that the primary engine is operated at the most appropriate torque/speed combination. Any surplus/deficit in power is then addressed with an electrical powertrain (energy storage, inverters and motors). One of the elegant ways this can be achieved is to use a “power-split” in what has become known as a “blended” electric hybrid as shown in FIG. 1. The power-split element 11 is a planetary gear 12 combined with a motor/generator 13. The motor/generator is used to control the rotation of an inner sun gear to vary the gear ratio of the planetary gear, while the engine 14 output and output to final drive are connected to the ring and planet carrier.
It should be well understood that a planetary gear has three elements. If any one of these elements is held static there is a fixed gear ratio between the other two elements and 3 different ratios can be achieved in any gear. By allowing the third element to rotate the ratio between the other two rotors will be varied.
By rotating and reacting torque on the sun gear the motor/generator imports or exports electrical power from the mechanical power train. For example, when there is a surplus of engine power above the required road load, power is extracted electrically and stored in the energy storage system 15, via an inverter 17. This energy may then be used at some later time either by the power-split motor returning power through the inverter 17 and through the power split system, or via a second inverter 18 and a secondary traction motor 16 driving the wheels. Therefore a “variator” path has been provided that allows engine power to reach the wheels through different paths and engine power requirement is to some extent decoupled from required road power.
These power-split systems can effectively give infinitely variable control of gear ratio, and can de-clutch engine to a point where engine has finite speed and the wheels have zero speed. This function has traditionally been achieved by the use of a disengaging clutch which itself is coupled to an inertial flywheel (on the ICE crank) which is also used to filter out torque pulsations from reciprocating engine. However, the flywheel reduces vehicle dynamic performance due to the added inertia. This clutch and flywheel may now be removed from the system and the engine crank would now be directly coupled to the power train through the power split device.
However, elimination of the clutch and adoption of direct coupling between engine and drivetrain leads to a more direct transmission of engine torque pulsations which would normally be largely attenuated by the clutch (through slipping/micro-movement) and inertial flywheel. This has impact on NVH (Noise, Vibration and Harshness) and hence “driver-feel” and wear on driveline components. This issue has been addressed with the use of Dual Mass Flywheels (DMFs).
Dual mass flywheels (DMFs) are usually fitted to diesel engine vehicles as they eliminate excessive transmission noise, protect the gearbox from damage and reduce gear change/shift effort. In a DMF, the mass of the conventional flywheel is split into two. One part is added to the engine's moment of inertia, while the other part increases the moment of inertia of the transmission. The two decoupled masses are linked by a spring/damping system and the DMF acts as a damper between the crankshaft and the input shaft on the gearbox. It also has a set of springs inserted between two rotating masses; the slip is cushioned by a set of torsional springs that smooth out irregular torque pulses from the engine. The springs are sized to absorb resonant vibration from the engine under load conditions.
This device introduces added cost and complexity to the drive train and has a number of wearing parts. Replacement is complex and costly due to its location between the engine and transmission.
Further, the above mechanical system suffers from a fundamental limitation of mechanical gears. The planetary gear must follow the big-wheel/small-wheel principle and the lowest torque, highest speed element is always the inner-most sun gear (while the ring gear and planet carrier are high-torque low-speed elements). It is advantageous to allow the mechanical power flow to be carried on the highest torque outer elements and drive the lowest torque highest speed element with the motor generator (as this will reduce the electrical machine size and increase its efficiency).
However, this leads to a complex shaft arrangement as access to the inner most gear is required, within the confines of two coaxial drive shafts.
A solution to these problems is desirable.